Field
The present application relates to adiabatically splitting and/or rotating the polarization of light.
Related Art
Waveguides are used in many applications to efficiently confine and guide electromagnetic radiation. Some waveguides are created using the principle of total internal reflection within a dielectric, or an approximation thereof. For example, in the field of integrated photonics, a first dielectric material of a first index of refraction is used to form a waveguide path and a dielectric of a second dielectric material, with a second index of refraction with a smaller value than the first index of refraction, is formed around the waveguide path. Using a dielectric material with a larger index as the waveguide path results in total internal reflection of the electromagnetic radiation.
When silicon waveguides with a small core (e.g., less than 1 micron) are formed on silicon-on-insulator (SOI) substrates, the waveguide tends to become birefringent. Thus, to ensure a photonic circuit is polarization independent, a polarization diversity scheme is conventionally used. Such schemes use polarization splitters and rotators to manipulate the polarization of the light within the photonic circuit. Also, many optical transmission formats utilize both polarizations, which generally requires transmitters and receivers capable of splitting and rotating polarization.
Conventional polarization rotators for use in a waveguide are based on the principles of operation of half-wave plates that are conventionally used in free-space optics. In free-space, half-wave plates formed from a birefringent material are introduced into the light path with an optical axis oriented at an angle with respect to the polarization of the light. Accordingly, the component of the light's polarization that is parallel to the optic axis experiences a first phase shift as it traverses the half-wave plate and the component of the light's polarization that is orthogonal to the optic axis experiences a second phase shift, different from the first phase shift, as it traverses the half-wave plate. The different phase shift between the two polarizations causes a rotation of the polarization of the light. Conventional polarization rotators for use in waveguides operate in the same way by introducing a birefringent waveguide into the path of the light.